1. Field of the Invention
The present invention relates to a fixing device used in an image forming apparatus utilizing an electrophotographic system, for example, a copier, a printer or a facsimile unit, as well as an image forming apparatus using the fixing device described above.
2. Prior Art
In a copier or printer utilizing the electrophotographic system, it is necessary to fix unfixed toner images formed on a recording sheet into permanent images, and a solvent fixing method, a pressure fixing method and a heat fixing method have been known for the fixing method.
However, the solvent fixing method involves a drawback that evaporating solvent vapors result in malodors or hygienic problems. On the other hand, the pressure fixing method involves a drawback that the fixing property is poor compared with other fixing methods and the pressure sensitive toner is expensive. In view of the above, both of the methods are scarcely put to practical use at present. Therefore, a heat fixing method of melting a toner by heating and heat fusing the same on a recording sheet has been adopted generally.
A heat roll type device has been known as a device by the heat fixing method, which comprises, as shown in FIG. 12, a fixing roller 121 having a heating source 123 in the inside of a cylindrical metal core 122 and a releasing layer 124 formed on the outer circumferential surface of the cylindrical metal core 122, and a pressure roller 125 having a cylindrical metal core 126, and a heat resistant elastic layer 127 and a releasing layer 128 made of a heat resistant resin film or a heat resistant rubber film formed on the outer circumferential surface of the cylindrical metal core 126. A recording sheet carrying unfixed toner images thereon (not illustrated) is passed between the fixing roller 121 and the pressure roller 125 to fix the toner images under heating and pressure. Since the heat roll system has a higher heat efficiency and suffers from less worry of paper burning caused by clogging compared with other heat fixing methods, for example, a heat radiation type fixing system, it has been utilized most generally at present.
The releasing layer 124 of the fixing roller 121 is made of a heat resistant resin film or heat resistant rubber film such that the toner images after fixing are easily stripped from the fixing roller 121. For the material of the releasing layer 124, it has been known generally that heat resistant resins typically represented by polytetrafluoroethylene (hereinafter referred to as "PTFE"), perfluoro alkyl vinyl ether polymer (hereinafter referred to as "PFA") and polytetrafluoroethylene hexafluoro propylene copolymer (hereinafter referred to as "FEP") can provide higher releasing performance without using a releasing agent (so-called releasing oil), compared with heat resistant rubber typically represented by silicone rubber or fluoro rubber. Accordingly, such heat resistant resins are generally used as the releasing layer in black and white copiers.
However, the fixing device in the full color copier or printer involves problems in view of the following points and the prior art described above cannot overcome such problems altogether.
1. Subject on Image Quality
When a fluoro resin is coated as a layer on the cylindrical metal core, since the material of the fluoro resin per se is rigid, this results in the following problems in view of the image quality.
FIG. 13 is an enlarged cross sectional view schematically showing the state of a fixing roller 131, a recording sheet 132 and a toner image 133 during fixing. As shown in FIG. 13, the toner image 133 has unevenness when viewed microscopically, so that it cannot follow after the fixing roller 131 if the roller is hard and adhesion in a microscopic state is deteriorated. Therefore, the toner image 133 after fixing causes minute unevenness of gloss in a solid area between a portion in contact with the fixing roller 131 and another portion not in contact therewith. Since the demand for the image quality in the black and white copier is not so high compared with that of the full color copier, use of the fixing roller comprising the metal core coated with the fluoro resin described above was satisfactory.
On the contrary, in the full color copier, demand for the image quality is much greater compared with that for the black and white copier. Accordingly, in the full color copier, the molecular weight of a binder resin is usually lowered so that the resin is easily deformable. This helps make the toner surface flat after fixing thereby attaining a high image quality. Furthermore, for suppressing unevenness of gloss, the cylindrical metal core is coated with heat resistant rubber, thereby, to improving the adhesion of the fixed image with the toner layer due to the stretchability of rubber per se and attaining excellent image quality with no unevenness of gloss.
2. Subject on Releasability and Reliability
A binder resin for the toner used in the black and white copier has a large molecular weight and shows elastic behavior. Therefore, the binder resin itself has good releasability with the fixing roller.
However, in the full color copier, a binder resin of lower molecular weight and showing strongly viscous behavior is used for the toner in view of the demand for the improvement of the image quality. Further, since the adhesion between the surface of the fixing roller and the toner is increased with an aim of eliminating unevenness of gloss, the area of contact between the fixing roller and the toner is enlarged to increase the deposition force. In the full color copier, since a toner of four colors, namely, yellow, magenta, cyan and black is used, the deposition force is further increased.
Therefore, the fluoro resin roller used in the black and white copier has poor releasability and cannot release the toner from that fixing roller.
In view of the above as disclosed in Japanese Published Unexamined Patent Application No, Hei 5-150679, it has been adopted a method of covering a fixing roller with heat resistant rubber and forming distortion in the fixing roller thereby obtaining high releasing performance. FIG. 14 is a side cross sectional view illustrating such a fixing device. A fixing roller 141 comprises a cylindrical metal core 142 and a releasing layer 144 made of a heat resistant resin film or a heat resistant rubber film formed on the circumferential surface of the cylindrical metal core 142. Reference numeral 145 denotes a pressure roller. In this fixing device, heating sources 146, 147 are disposed at the inside in both of the fixing roller 141 and the pressure roller 145.
"Distortion" means herein distortion formed on the surface of the heat resistant rubber layer 143 when the pressure roller 145 is in press contact with the fixing roller 141 and the heat resistant rubber layer 143 of the fixing roller 141 deforms elastically.
The fixing roller as disclosed in Japanese Published Unexamined Patent Application No. Hei 5-150679 has a structure in which an LTV silicone rubber layer is formed on a cylindrical metal core and, an RTV silicone rubber layer is formed as a releasing layer further thereon. Since the RTV silicone rubber layer per se at the outermost layer has an expandability, the distortion tends to be formed.
However, releasability of rubber itself is not so high and, in order to ensure sufficient releasability, a releasing oil has to be coated or supplied separately to the releasing layer of the fixing roller. That is, when the releasing oil is coated, the toner is released from the fixing roller due to the intra-layer separation of the oil layer. At present, a fixing device having a rubber roller using such a releasing oil has been adopted in most of full color copiers.
However, since the releasing oil is used, this has resulted in problems such as swelling of the silicone rubber with oil, leakage of the oil in the device, deterioration of easy writing by a ball point pen or ink caused by the oil on the recording sheet after fixing. Further, maintenance such as periodical supplement of the oil is also necessary, which is not suitable to a small-sized full color copier or a printer. Further, since the friction coefficient of the silicone rubber is high, there are also problems of abrasion in the releasing layer by the passage of sheets or deterioration in the releasability caused by the denaturation of the silicone rubber per se.
Accordingly, it is considered that if a fluoro resin having high releasability by itself is coated on the heat resistant rubber layer, high releasability can be obtained and no additional releasing oil coating unit is necessary. Namely, it is considered that an excellent image quality can be obtained together with high releasability by use of a fixing roller comprising heat resistant rubber on which a fluoro resin is dispersion-coated and baked as disclosed in, for example, Japanese Published Unexamined Patent Application No. Sho 61-22376 and Japanese Published Unexamined Patent Application No. Sho 61-248731. However, since the coated and baked film as disclosed above has a thickness of about 2 to 3 .mu.m and since pressure by the pressure roller is high, it results in a problem that the releasability is lowered by the frictional abrasion of the releasing layer caused by the passage of sheets.
The problem of the frictional abrasion can be overcome by covering the surface of the heat resistant rubber with a previously prepared fluoro resin tube as disclosed, for example, in Japanese Published Unexamined Patent Application No. Sho 57-89785, Japanese Published Unexamined Patent Application No. Sho 53-114474, Japanese Published Examined Patent Application No. Hei 7-349, and Japanese Published Unexamined Patent Application No. Hei 4-42183. However, since the thickness of a usual fluoro resin tube is from about 50 to 100 .mu.m, the tube itself is rigid and it is difficult to form the distortion described above and a purpose of attaining high releasability and an excellent image quality cannot be attained satisfactorily.
Further, if a fluoro resin layer is disposed on the surface of the heat resistant rubber to provide further distortion, the releasability can be improved. However, since the shrinkage is different between the rubber and the resin, if large distortion is given, the fluoro resin causes plastic deformation to result in creasing. Namely, in the existent fixing device using the heat/pressure roller pair, since stripping is conducted by providing a relatively thick rubber layer with large distortion, if a fluoro resin layer is additionally disposed, it may possibly result in creasing. Since the fluoro resin layer tends to cause more creasing as the layer thickness is reduced, the problem of creasing becomes conspicuous if it is intended to use a thinner fluoro resin layer in order to improve the image quality and the releasability.
3. Subject of Energy Saving and High Speed Fixing
Further, when the fixing roller 141 and the pressure roller 145 are brought into press contact with each other, to ensure a wide nip and form large distortion as in the fixing device shown in FIG. 14, it is necessary to increase the wall thickness of the cylindrical metal core 142 of the fixing roller 141 and also increase the wall thickness of the heat resistant rubber layer 143 disposed thereon. Accordingly, this necessarily increases the heat capacity of the fixing device and it takes about three to six minutes as a temperature rising time from a room temperature. Therefore, it is necessary keep the fixing device at a somewhat high temperature also in a stand-by state in which images are not formed, and a most portion of the electric power consumption in the copier or the like is caused by this preheating at present.
Further, presence of a thick rubber layer having low heat conductivity gives extremely high heat resistance from the inner circumferential surface to the outer circumferential surface of the fixing roller 141. Accordingly, even if the heater 146 in the fixing roller 141 is heated, the heat is not conducted easily to the outer circumferential surface of the fixing roller 141. This gives a hindrance in the increase of the operation speed of the fixing device.
A fixing device having a rapid temperature rising characteristic (hereinafter referred to as "instant starting performance") by using a thin film and a fixed heater has already been disclosed (although in a black and white fixing device) in, for example, Japanese Published Unexamined Patent Application Nos. Sho 63-313182 and Hei 4-44074. However, the technique cannot provide high image quality and high speed adaptability to operation required for full color fixing. This is because the fixing device described just above adopts a system of conducting heat from the heater to a recording sheet by way of the thin film having a heat capacity substantially equal with zero, so that heat conduction is worsened if an elastic layer is disposed on the film for improving the image quality and, after all, the device cannot be adopted in a system.
As described above, respective problems have been solved individually but a fixing device capable of satisfying the problems altogether has not yet been attained.